Despite considerable advances in wireless transceiver technology, receiver and transmitter nonlinearity continues to limit the performance of wireless systems and devices. In the design of radio receivers, nonlinearity restricts the ability of a radio to receive weak signals in the presence of nearby stronger signals. In radio transmitters, nonlinearity can cause the transmitted signal to spill over into adjacent frequency channels, interfering with other users.
Modern mobile communication systems use Orthogonal Frequency-Division Multiplexing (OFDM) schemes to transmit multiple channels, closely spaced over an assigned frequency band. OFDM is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data are divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme, such as quadrature amplitude modulation or phase shift keying.
An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent phases of the sub-carriers often combine constructively. Handling the high PAPR requires a high-resolution digital-to-analog converter (DAC) in the transmitter, a high-resolution analog-to-digital converter (ADC) in the receiver, and a linear signal chain.
The linearity requirement is problematic because amplifiers used in RF circuits are inherently non-linear in order to minimize power consumption. Nonlinearity in the signal chain causes signal compression and as a result introduces distortion and intermodulation distortion. Nonlinearities are more pronounced at higher power levels, with the rate of errors in a wireless channel being related to average power, among other variables. An amplifier maintains a constant gain for low-level input signals. However, at higher input levels, the amplifier goes into saturation and its gain decreases. A 1 dB compression point (G1dB) indicates a power level that causes the gain to drop by 1 dB from its small signal value. To reduce the signal compression, the operating point of the amplifier can be set far from the compression point which in turn causes low power efficiency.
As is well known, a major source of non-linearity is distortion and spectral regrowth, which occur due to non-linear amplitude and phase response of the amplifier, particularly as power nears the saturation level. Third-order distortion nonlinearities typically cause the strongest intermodulation products.
Most emerging OFDM and WCDMA standards produce high peak-to-average ratio signals. In the past, when most applications produced a low peak-to-average ratio signals, nonlinearities at high power levels did not have important effect. However, as required peak-to-average ratio increases, intermodulation and spectral regrowth created by the RF front end (mainly by the power amplifier) become unavoidable.
To linearize amplifier output, compensation methodologies have been devised. One compensation method is predistortion, which entails inserting a nonlinearity (i.e., non-linear amplitude and phase characteristics complementary to the distortion) prior to a radio frequency power amplifier (PA) such that the combined transfer characteristic is linearized, as disclosed in U.S. Pat. Nos. 5,236,837 and 6,240,278. Ideally, the predistorter cancels distortion in the amplifier output over the entire signal bandwidth.
Post-distorters have also been used, mainly to reduce the calculation complexity of a predistorter. By way of example, US Patent Publication No. 2004/0032297 to Nygren discloses a post-distorter adapted to calculate system identification coefficients, and then substitute the calculated coefficients in a predistorter positioned before the power amplifier. Another known use for post-distortion is to simply decrease the bit error rate (BER) by correcting the transmitter power amplifier at the receiver chain, as disclosed in US Patent Publication No. 2004/0196921 to Matsumoto.
When a high peek to average ratio (PAR) signal is transmitted via power amplifier, the transitory temperature rises very quickly as the peek is reached. For a brief period of time (e.g., a couple of microseconds) the transistor temperature increases significantly. For example, if the peak to average ratio of a signal is 6 dB, the heat generated by the transistor may be 4 times higher the average. This phenomena causes the power amplifier to operate at a different power curve than normal. This is referred to as a “memory” effect, because the histogram of the power amplifier is related to the characteristic history of the signal being transmitted. The thermal resistance and capacitance of Silicon is a key factor in the memory effect. Additionally, for an RF-amplifier using a high PAR signal, the 3rd-order intermodulation distortion caused by the CMOS thermal memory effect is severe.
Various schemes have been proposed for digital-domain predistortion of RF power amplifier input signals. For example, U.S. Pat. No. 6,141,390, describes a system that uses a straight inverse modeling scheme with orthogonal predictor variables to determine the inverse of the distortion caused by a power amplifier of a RF transmitter. The predistorter determines complex predistorter coefficients based on the inverse modeling scheme, and stores the coefficients in a look-up table (LUT). The coefficients from the LUT are then used as the tap weights of a non-linear digital filter implementing the predistorter.
Other digital predistortion systems as described in U.S. Pat. Nos. 6,549,067 and 6,580,320, sample input to a non-linear amplifier and multiplies the input by itself using mixers in order to generate various orders of distortion. Filters/time-delay means are incorporated into the paths that generate the orders of distortion in order to produce phase and/or amplitude variation with frequency. The distortion orders are summed to provide the predistortion. The filter/time delay means can be implemented by adaptive filters in digital signal processing (DSP) circuits, which sample the output of the amplifier being linearized in order to obtain feedback for adapting the filter(s). The in-phase and quadrature parts of the input are separately digitally predistorted.
Most predistorter systems have two modes. During a calibration mode, the predistorter mechanism calculates system identification coefficients. During a data transmit mode (normal transmit mode using the calculated coefficients) the calculated coefficients are applied to negate nonlinearities. These prior art approaches erroneously assume that the receiver is linear both in calibration mode and in data transmit mode.
Despite the aforementioned advances, no known prior art post-distortion methodologies are designed to correct the receive path nonlinearity, particularly during calibration. Many transmitters are positioned adjacent to a receiver, usually in the same single chip. The power amplifier broadcasts substantial power, sometimes more than 30 dBm (1 Watt) to a −100 dBm sensitive receiver, thereby compressing the receiver.
The predistorter cannot be calibrated if the receive chain is saturated while in calibration mode. If calibration is attempted while the receiver is in saturation, the result will be a substantial degradation of predistorter performance.
One attempt to solve this problem is to add a secondary receiver as a low sensitivity receiver that will not be compressed. However, this approach is costly and inefficient. Another popular approach is to shut down the power supply to the low noise amplifier (LNA) while in calibration mode, thus making it act as an attenuator. Nevertheless, the receiver still exhibits some nonlinearity due to saturation.
What is needed is a digital solution that compensates for RF front-end nonlinearities using predistortion and post-distortion to provide correction for both the receiver and the transmitter in a calibration mode and that obviates a secondary receiver. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.